Extending an existing wall involves drilling through a well lining and then lowering a well lining extension into the drilled bore. Cement can then be poured into the space between the walls of the bore and the well lining extension to form a permanent concrete/cement well lining. A variety of methods exist for filling the space between the walls of the bore and the well lining extension. One method as disclosed in U.S. Pat. No. 5,277,244 to Bell involves attaching a well liner running shoe to the bottom of the well liner extension and then using the working string to pump cement down the well liner. A one way valve is formed in the running shoe in the Bell patent to prevent the backflow of cement into the well lining. While the running shoe disclosed in Bell is suitable, it is fairly complex to build and operate. An improved valve/tie back float collar which overcomes the disadvantages of the Bell device is therefore required.